Fuel cells based on hollow conductive carbon fibres

ABSTRACT

An improved fuel cell structure uses a first sheet of carbon fibres separated by an electrolyte from a second sheet of carbon fibres. A fuel cell catalyst is coated on the outside of the fibres and the fuel passed down one set of fibres and oxygen or air passed down the second sheet of fibres.

The present application is a continuation of application Ser. No.10/362,377 filed on Sep. 5, 2003 which is hereby incorporated byreference herein.

The present invention relates to a fuel cell, more particularly itrelates to an improved fuel cell configuration.

In a basic polymer electrolyte membrane (PEM) fuel cell which useshydrogen as a fuel, the fuel cell comprises anode and cathode bipolargraphite plates, which control the addition of the feed hydrogen andoxygen to the cell. In addition the cell has two gas permeable sheets ofcarbon paper which are coated on their inside with a catalyst, which areseparated by a proton conducting membrane and which are impregnated withPTFE in order to control moisture penetration. The hydrogen is passedinto the cathode chamber and oxygen is passed into the anode chamber.When the external circuit is completed by externally electricallyconnecting the cathode and anode, hydrogen passes through the PTFEimpregnated carbon paper layer and is ionised by the catalyst. Theprotons formed then pass through the polymer electrolyte membrane (PEM)to the oxygen electrode where they combine with oxygen ions to producewater and electrons. The water generated is then primarily removed inthe air stream.

To date the polymer electrolyte membranes (PEM) have usually been basedon a fluorinated polymer made by Du Pont and sold under the Trade Name“Nafion”. This basically consists of tetrafluorethylene withperfluorovinyl monomers having sulphonate functional groups allowing forthe conduction of protons to provide the functional groups. This polymercurrently gives the best mix of mechanical strength and protonconductivity properties essential for the construction and operation ofthe cell although a wide variety of other polymers have been and are nowbeing investigated, these include, DOW's XUS-13204.10 membraneelectrolyte, Dais 585 (a sulphonated styrene-ethylene, butylene-styrenetriblock copolymer) and Flemion (a carboxylic membrane).

The catalysts commonly used are platinum for the cathode and a platinumalloy such as platinum-ruthenium and platinum-molybdenum for the anode.

Generally the shorter the path length for the gas molecules through thecarbon paper/PTFE catalyst support assemblies and the shorter the pathlength for the protons through the PEM the more efficient the cell willbe as high transport resistances lead to a significant fall in cellvoltage.

Operation at high current density, as is planned for commercialoperation (typically in excess of 1000 ma/cm²) will lead to a reductionin cell voltage to around 0.6V with the lower efficiency leading to ahigher heat release from the cell.

The various components of fuel cells have to have a minimum thicknessowing to the need for there to be coherent layers and for handle-abilityduring cell assembly. This means that for existing cells the pathlengths through the various components of a fuel cell cannot be reducedbelow a minimum value. In particular, the thickness of the graphitebipolar plates is controlled by the need for the complex gas flowchannels, which both transport the feed gases into the cell and have toremove the liquid water produced in the cell reaction, and by themechanical properties of the graphite. At present thickness of less thanaround 1-2 mm are unlikely to be attainable. The thickness of the carbonpaper layer (which is also impregnated with PTFE to control wetting) hasbeen progressively reduced to around 100-300 microns but is unlikely toreduce significantly further due to mechanical constraints whilst thePEM/catalyst layer thickness (membrane electrode assembly) seems to beapproaching a minimum at around 200 microns which includes the 10 micronthick Platinum/carbon “ink” layer on each side of the polymerelectrolyte. This gives an overall thickness for a single cell of around2500-3000 microns.

The platinum loadings in the PEM cells has been progressively reducedfrom around 4 mg/cm² to 0.5 mg/cm² in commercial cells with somelaboratory cells using as little as 0.15 mg/cm². The platinum is appliedas an ink formed from platinum impregnated carbon which is “painted” or“printed” onto either side of the polymer electrolyte in a layer around10 microns thick.

Another factor is the available surface area of catalyst per unit volumeof the cell, with a larger surface area for a given size of cell alarger current can theoretically be generated and the better theperformance of the cell will be, but this is limited by the geometricsize of the anode and cathode assemblies.

The individual cells are bipolar—that is to say that, in use, the cellsare stacked together with each bipolar plate forming the negative sideof one cell and the positive side of the adjacent cell. The currentproduced is carried directly through the cell without the requirementfor any external connections—alternate sides of the bipolar graphiteplates are negative and positive with hydrogen being fed to one side ofthe plate and oxygen to the other. This demonstrates two furtherconstraints on the bipolar plates—they must be totally gas impermeableand must also have a very low electrical resistance across the plate.

In order to improve the performance of the basic fuel cell a largenumber of incremental improvements have been patented over the last 20years which have increased the cell power density from around 0.1 W/cm³in 1990 to the 1.4 W/cm³ that is claimed for the latest cells.

The PTFE impregnated carbon paper combined with the catalyst layers andthe proton conducting polymer layer is referred to as themembrane/electrode assembly. The evolution of membrane/electrodeassemblies in polymer electrolyte membrane fuel cells has passed throughseveral generations. The original membrane/electrode assemblies wereconstructed in the 1960s for the Gemini space program and used 4milligrams of platinum per square centimetre of membrane area (4mg/cm²). Current technology varies with the manufacturer, but totalplatinum loading has decreased from the original 4 mg/cm² to about 0.5mg/cm². Laboratory research now uses platinum loadings of 0.15 mg/cm².The thickness of the membrane in a membrane/electrode assembly can varywith the type of membrane. The thickness of the catalyst layers dependsupon how much platinum is used in each electrode. For catalyst layerscontaining about 0.15 mg Pt/cm², the thickness of the catalyst layer isclose to 10 microns.

The central layer of the membrane/electrode assembly comprises a thin,˜50-200 micron, sheet of Nafion conducting polymer. The thickness ofthis layer has a major impact on the overall cell performance but, giventhe way the cells are constructed, it cannot be practically reduced toomuch below the current thickness. There is a large amount of workcurrently underway to find alternatives to Nafion to reduce the cellcost but finding a polymer with the required proton conductivity andacceptable mechanical properties has proved difficult.

The cell performance is limited by the diffusion of the water and thegases through the membrane layers as well as the electrical performanceof the Pt and Nafion layers. Any reduction in the thickness of thevarious transport layers can significantly improve the overall masstransfer performance of the cell however with this type of cell,although widely used it is difficult to obtain a substantial improvementin performance.

Fuel cells are described in a large body of literature including T RRalph et al, 1997, J Electrochem Soc, 144, 11, 3845; T E Springer et al,1991, J Electrochem Soc, 138, 8, 2334; D M Bernardi and M W Verbrugge,1992, J Electrochem Soc, 139, 9, 2477.

We have now devised a radically different design of fuel cell whichovercomes many of the limitations imposed by the conventional bipolarplate PEM cell design.

According to the invention there is provided a fuel cell which comprisesa first hollow electrically conductive gas permeable fibre which iscoated on the outside with a fuel cell catalyst and which has a means toenable a fuel vapour or gas to be passed down the inside of the fibreand a second hollow electrically conductive gas permeable fibre which iscoated on the outside with a fuel cell catalyst and which has a means toenable oxygen or an oxygen containing gas to be passed down the insideof the fibre, the first and second carbon fibres being separated by theelectrolyte medium, which may be a solid polymer electrolyte such asNafion or a liquid or gel electrolyte, and preferably by a polymericmesh to ensure that the fibres do not touch which would give rise to ashort circuit.

Any liquid or vapour fuel used in fuel cells can be used in the presentinvention e.g. hydrogen, methanol, etc.

In use the fuel gas or vapour e.g. hydrogen or methanol is passed downthe inside of the first hollow fibre (cathode) and oxygen or an oxygencontaining gas is passed down the inside of the second (anode) hollowfibre. The gases pass through the walls of the hollow carbon fibres. Thehydrogen ionises to produce a proton and an electron at the catalyst atthe cathode fibre surface and the proton diffuses through theelectrolyte to the cathode fibre. At the anode fibre the oxygen isionised at the catalyst at the fibre surface whereupon it reacts withthe proton to complete the electrochemical reaction and generate anelectric current as in conventional fuel cells. The anode and cathodecatalysts can be selected from those materials known to work effectivelyin the respective anode and cathode environments.

Preferably there are a plurality of first and second hollow electricallyconductive gas permeable fibres arranged with all the first hollowelectrically conductive gas permeable fibres arranged side by side,preferably in rows, so as to form a first sheet of the fibres, and withthe second hollow electrically conductive gas permeable fibres similarlyarranged to form a second sheet of the second hollow electricallyconductive gas permeable fibres. The fibres in each adjacent row arepreferably at an angle to each other and are preferably substantiallyorthogonal to each other.

The electrically conductive gas permeable fibres in each layer arepreferably aligned with their axes substantially parallel and may be incontact.

The first electrically conductive gas permeable fibre layer (anode) andthe second electrically conductive gas permeable fibre layer (cathode)must be separated by the fuel cell proton conducting layer which ispositioned in between them. The layers are also preferably separated byan electrically insulating polymeric mesh which maintains the requiredseparation between the anode and cathode layers and prevents anelectrical short between the layers.

However in the structure of the present invention it is not necessary tohave a solid membrane, such as the Nafion sheet used in conventionalfuel cells, and the fibres can be separated by any means of separationwhich enables a flow of protons to take place. The structure of thepresent invention enables the fibres to be fixed in position with asmall gap between them so that they do not come into contact but thereis a short path through the electrolyte between them. The electrolytecan be a liquid, gel or solid polymer.

The individual fibres in each layer are be electrically connected toeach other at one or preferably both ends e.g. by means of a conductorplate or block. This conductor plate or block can be formed from forinstance silver loaded epoxy adhesive as used in electrical circuits andis commercially available. Alternatively the ends of the fibres can becopper or nickel plated using either electroplating or electrolessplating and then soldered together. In either case the conducting blockso formed can contain a metal mesh that provides the means for makingthe external electrical connection to the cell.

A multilayered block can be formed in which there are a number ofalternate layers comprising a layer of the first electrically conductivegas permeable fibres forming cathodes and a layer of the secondelectrically conductive gas permeable fibres forming the anodes with thefibres in each alternate layer substantially orthogonal to each other.The layers are separated as referred to above. As well as the cathodefibres being connected together and the anode fibres being connectedtogether in each block plurality of blocks can be connected together inseries or parallel depending on the voltage and current requirements.Such a multi cell arrangement can be preferentially formed within asingle polymer block which contains all of the necessary gasinterconnections such that only a single set of hydrogen or oxygenconnections are required irrespective of the number of cells. The meansfor electrically interconnecting the cells can also be incorporatedwithin the block through for instance a printed circuit array, built forinstance into the top or base of the block, that allows any mixture ofseries and parallel interconnections to meet any given voltage-currentrequirement.

The electricity generated is conducted away from the fuel cell structureby a conductor attached to the ends of the hollow carbon fibres usingconventional contacts. As the electric current generated is taken fromthe fuel cell by a conductor electrically connected to the hollowfibres, the axes of the hollow carbon fibres forming the anodes andcathodes being substantially orthogonal to each other facilitates theleading away of the generated electric current.

The fibres in each layer being aligned orthogonally to the fibres in theadjacent layer also facilitates the feeding of the gases to each layerof fibres and the removal of water vapour and any other gases or vapoursgenerated when a fuel other than hydrogen is used.

We have also found that due to the small size of the cell heat removalis straightforward and there is no requirement for additional coolingfacilities other than perhaps a finned array, as used to cool computerchips, on the upper and/or lower surfaces of the cell block. This canalso if required be further cooled by a fan array as used with computerprocessor chips.

The outside diameter of the individual hollow fibres is preferably from80 to 1500 microns e.g. 80 to 500 microns and more preferably from 100to 300 microns. The wall thickness of the hollow carbon fibre ispreferably from 10 to 200 microns e.g. 10 to 100 microns and morepreferably from 20 to 100 micron. The inside diameter of the individualfibres is preferably from 150 to 1300 microns, more preferably from 70to 250 microns.

The hollow fibres carry out the combined functions of the graphiteplates and the carbon paper assemblies in the conventional fuel cellsdescribed above. As such the fibres should have sufficient conductivityto carry the current generated without an excessive ohmic loss. Theelectrical conductivity of the fibres can be enhanced by hightemperature heat treatment although care must be exercised that suchtreatment does not excessively reduce the gas permeability of the fibrewalls. In the event that the permeability is reduced excessively by heattreatment the porosity and permeability can be restored by carefuloxidation of the fibre. This is preferentially carried out using hightemperature steam or carbon dioxide to ensure that the activating gaspenetrates through the fibre.

The anode and cathode catalysts can be deposited within the outersurface of the carbon fibres by conventional catalyst preparationtechniques, but preferentially uses methods that do not require heatingto reduce the active metal components. Such methods include for instanceion exchange followed by reduction using e.g. formaldehyde orelectroless plating. The surface of the carbon fibres is preferentiallytreated prior to ion exchange to enhance the acidity and ion exchangecapacity. Typical methods include for instance mild air activation ortreatment with nitric acid. Careful air activation has the benefit oflimiting the development of the acidity to the surface region of thefibre which helps to ensure that the catalytic metal is predominantlypresent at the surface of the fibre rather then being uniformlydeposited throughout the fibre. Care must be taken during these stagesas the nature of the carbon fibre and the way in which the catalyst isadded will affect the hydrophobic characteristics of the fibre porestructure. It is not anticipated that PTFE modification will be requiredto control the water permeation characteristics of the fibres althoughother chemical treatments, such as bromination could be used to reducewater permeation.

The fuel cell catalyst is not critical and any fuel cell catalyst can beused in the present invention which will work with the fuel used. Mostcatalysts are based on platinum or platinum group metal catalysts and avery large number of catalyst compositions have been proposed.

The length of the fibres is fixed by the fibre resistivity and thepermitted level of ohmic losses in the cell. The optimum cell dimensionwill be a function of the fibre resistivity, the fibre dimensions andthe cell operating characteristics. The high volumetric efficiency ofthe hollow fibre design permits the operation of the cell at much lowercurrent densities (amps/cm²) than is normally permitted, and at thelower power densities high fibre resistivities or longer fibre lengthscan be tolerated. The most efficient cell design will use the maximumpossible fibre length, subject to ohmic loss constraints, as this willminimise the number of fibre end connections that will be necessary.When air is used as the oxidising agent the volume of air required ismuch larger than the volume of a fuel such as hydrogen flowing down thefuel containing fibres and this could lead to a cell in which the fibrescarrying the air are of a larger diameter than those carrying thehydrogen fibre length is 1 cm or greater e.g. 5 cm and has a surfacearea of about 0.6 cm².

Proton conducting materials which can be used in the fuel cell structureof the present invention include any known proton conducting materialand is not limited to the solid polymer electrolyte membranes used inconventional cells. Possible electrolytes include those used anddisclosed for use in fuel cells.

In traditional bipolar plate PEM fuel cells, complete electrodestructures are usually developed either (i) by using preformed electrodestructures that contain a carbon supported catalyst andpolytetrafluoroethylene and then incorporating a soluble electrolytecomponent in the final processing step or (ii) by premixing together theelectrocatalyst and the soluble polymer components and applying them ina single coating step. Such electrode structures are then usually‘printed’ onto the carbon fibre substrate to form the complete electrodestructure (reference, Ralph et al, 1997, J Electrochem Soc, 144, 11,3845). In this invention, the catalyst will be preferentiallypre-impregnated on to the carbon fibres and the carbon fibres will thenbe assembled after which the free space in between the fibres will befilled with a polymer conducting membrane.

When a solid “Nafion” type membrane is used the fibres in each layer areseparated by the membrane and a block structure can be formed in whichthe fibres in each layer are separated from the fibres in the adjacentlayer and held in position by the membrane. This structure can be formedplacing the membrane between the layers of first and second hollowfibres and hot pressing the structure to form a compact structure.Alternatively the polymer can be added by e.g. solution casting. In thiscase it is preferable although not essential to use the polymer meshseparator. In the case of liquid or gel electrolytes it will benecessary to use the polymer mesh separator.

The carbon fibres used in the present invention can be made by knownmethods e.g. from polymer precursors.

The invention provides a fuel cell structure which is compact and givesa higher power/weight ratio and higher power/volume ratio than existingfuel cells. This makes it ideal for use in for instance portableelectrical devises such as computers, power tools etc.

The carbon based fibre cell of the present invention will allowoperation at lower current densities, potentially as low as 100-200mA/cm², and therefore higher efficiency, without compromising overallvolumetric efficiency.

The invention is described with reference to the accompanying drawingsin which FIG. 1 relates to prior art fuel cells and in which:

FIG. 1 shows a diagrammatic view of a cell of bipolar plate PEM assembly

FIG. 2 shows a schematic view of part a fuel cell according to theinvention showing the arrangement of the fibres

FIG. 3 is a perspective view of one embodiment of the invention

FIG. 4 is a perspective view of another embodiment of the invention

FIG. 5 is diagram showing gas flow through a cell block

FIGS. 6 and 7 are diagrams showing electrical connections betweenindividual cells

FIG. 8 shows a plan of one layer in a cell block and

FIG. 9 is an exploded view of a fuel cell assembly

Referring to FIG. 1 the cell comprises a bipolar plate (41), a carbonPTFE paper (42) a Nafion polymer membrane (43) and platinum “ink” (44).Bipolar graphite plates (41) which are thin graphite plates, withcomplex shaped pathways machined into both of the surfaces to maximisegas distribution to the catalyst layer. These plates are of the order of2 mm thick. The catalyst layer comprises a carbon paper (42) impregnatedwith PTFE to control water flow and the level of hydration of theelectrolyte and impregnated with the electrode catalyst at the surfacein contact with the proton conducting membrane. The catalyst comprisesplatinum, platinum group metals or mixtures of these in the form ofhighly dispersed platinum impregnated into the carbon paper (44). Thispaper layer is typically around 200 microns thick

The main feed gases, hydrogen and oxygen, are fed to each side of theplate.

Referring to FIG. 2 the fibres and tubes are embedded in a Nafionpolymer shown generally at (11) and the platinum coating on the fibres(1) and (3) is shown at (10 a) and (10 b). The oxygen or air carryingflows down the passageway (1 a) in the direction of the arrow. Theoxygen can diffuse through the walls of the fibre (1) to the platinumlayer (10 a). In the hydrogen carrying fibres (3) the hydrogen will passdown the passageway (3 a) and can diffuse through the walls of the fibre(3) to the platinum layer (10 b). Coolant will pass down the metal tubes(2) in the direction of (2 a).

In use the Nafion membrane is wetted with water and hydrogen is passeddown fibres (3) and oxygen or air passes down the fibres (1), when thehydrogen diffuses through fibres (3) it is ionised at the platinum layer(10 b)/Nafion interface to give H⁺ ions and when the oxygen diffusesthrough fibres (1) it is ionised platinum layer (10 a)/Nafion interfaceto give OH⁻ ions, the H⁺ and the OH⁻ ions combine to form water andgenerate electric current the water mainly diffuses through fibres (3)and is removed in the air or oxygen stream.

The ends of the fibres (1) are connected together and the ends of thefibres (3) are connected together by conductive plates, to enable therequired voltage to be produced by the cell block.

Referring to FIG. 3 a number of fibres (1) are arranged side by side inrows so as to form a first sheet of the fibres, the hollow fibres (3)are similarly arranged to form a second sheet and the fibres in eachsheet are orthogonal to the fibres in the adjacent sheet. The oxygen orair flows down the fibres (1) and the fuel e.g. hydrogen flows downfibres (3) as described in FIG. 1 and the ends of the fibres (1) aresupported in conducting block (14). The fibres (1) and (3) are keptapart by means of insulating mesh (12) so that a gel electrolyte can beused in place of the “Nafion” membrane shown in FIG. (1).

Referring to FIG. 4 the rows of fibres are kept apart by insulatingpolymer mesh (15).

Referring to FIG. 5 this shows diagrammatically two rows of fibres (16)and 917) with the fibres in row (16) being fibres (1) if FIG. 2 or 3 androw (17) being fibres (3) of FIG. 2 or 3. The ends of each row of fibresare electrically connected together and held in a block (18). To build acell block several pairs of rows are assembled together as shown in FIG.6 in which a block of assembled fibres shown in FIG. 5 are held in astack.

Referring to FIG. 7 this shows six cell blocks connected together andshows the gas flows through the block. Hydrogen fuel enters at entrance(19) follows the path shown and exits at (20). Air enters at (21), flowsthe path shown and exits at (22). The hydrogen and air flow down theirrespective fibres as described above.

FIGS. 8 and 9 show two different electrical connections for the block ofcells of FIG. 7.

The arrangement is chosen depends on the voltage and currentrequirements and resistivity of the fibres.

Power densities for fibre cells are given in the table.

The power densities, in W/cm³, for the fibre dimensions given in theTables were determined using an assumed current density, mA/cm², theavailable fibre surface area in a given cell volume and the expectedoperating voltage. The available fibre surface area was assumed to bethe external surface area of a fibre and the expected operating voltagewas calculated as the difference between the ideal cell voltage (0.75)and the expected losses. The expected voltage losses are ohmic losseswhich are an integrated function of fibre cross-sectional area, lengthand the material resistivity. Three cases are shown in the Tables, theserepresent different assumed resistivities for the carbon fibres, 1×10⁻⁶,1×10⁻⁵, and 1×10⁻⁴ ohm m. This range of resistivity covers that which ismost expected for the carbon fibres that will be used in this improvedfuel cell configuration. od is the outside diameter of the fibre and idis the inside diameter of the fibres.

TABLE 1 wall fibre current power ohmic fibre od thickness fibre idlength density density voltage operating microns microns Microns micronsmA/cm² W/cm³ loss voltage 200 25 150 1.0 100 10.41 00024 0.748 300 25250 1.0 100 7.22 0.0023 0.748 200 50 100 1.0 100 10.42 0.0014 0.749 30050 200 1.0 100 7.23 0.0012 0.749 200 25 150 1.0 200 26.02 0.0024 0.748300 25 250 1.0 200 18.05 0.0023 0.748 200 50 100 1.0 200 26.06 0.00140.749 300 50 200 1.0 200 18.07 0.0012 0.749 400 50 300 1.0 500 27.660.0012 0.749 800 50 700 1.0 500 14.26 0.0011 0.749 400 100 200 1.0 50027.68 0.0007 0.749 800 100 600 1.0 500 14.27 0.0006 0.749 400 50 300 1.0750 41.49 0.0012 0.749 800 50 700 1.0 750 21.39 0.0011 0.749 400 100 2001.0 750 41.52 0.0007 0.749 800 100 600 1.0 750 21.41 0.0006 0.749 1000100 800 1.0 750 17.23 0.0006 0.749 1500 100 1300 1.0 750 11.58 0.00060.749 1000 200 600 1.0 750 17.24 0.0003 0.750 1500 200 1100 1.0 50 11.590.0003 0.750 1000 100 800 1.0 900 20.68 0.0006 0.749 1500 100 1300 1.0900 13.90 0.0006 0.749 1000 200 600 1.0 900 20.68 0.0003 0.750 1500 2001100 1.0 900 13.90 0.0003 0.751 Fibre resistance = 1 × 10⁻⁶ ohm m

TABLE 2 wall fibre current power ohmic fibre od thickness fibre idlength density density voltage operating microns microns Microns micronsmA/cm² W/cm³ loss voltage 200 25 150 1.0 100 10.11 0.0238 0.726 300 25250 1.0 100 7.02 0.0227 0.727 200 50 100 1.0 100 10.25 0.0139 0.736 30050 200 1.0 100 7.12 0.0125 0.738 200 25 150 1.0 200 25.28 0.0238 0.726300 25 250 1.0 200 17.56 0.0227 0.727 200 50 100 1.0 200 25.62 0.01390.736 300 50 200 1.0 200 17.80 0.0125 0.738 400 50 300 1.0 500 27.270.0119 0.738 800 50 700 1.0 500 14.07 0.0111 0.739 400 100 200 1.0 50027.45 0.0069 0.743 800 100 600 1.0 500 14.17 0.0059 0.744 400 50 300 1.0750 40.90 0.0119 0.738 800 50 700 1.0 750 21.11 0.0111 0.739 400 100 2001.0 750 41.18 0.0069 0.743 800 100 600 1.0 750 21.25 0.0059 0.744 1000100 800 1.0 750 17.11 0.0058 0.744 1500 100 1300 1.0 750 11.51 0.00560.744 1000 200 600 1.0 750 17.17 0.0033 0.747 1500 200 1100 1.0 50 11.550.0030 0.747 1000 100 800 1.0 900 20.53 0.0058 0.744 1500 100 1300 1.0900 13.81 0.0056 0.744 1000 200 600 1.0 900 20.60 0.0033 0.747 1500 2001100 1.0 900 13.85 0.0030 0.747 Fibre resistance = 1 × 10⁻⁵ ohm m

TABLE 3 wall Fibre current power ohmic fibre od thickness fibre idlength density density voltage Operating microns microns Microns micronsmA/cm² W/cm³ loss Voltage 200 25 150 1.0 100 7.13 0.2377 0.512 300 25250 1.0 100 5.05 0.2269 0.523 200 50 100 1.0 100 8.51 0.1387 0.611 30050 200 1.0 100 6.04 0.1248 0.625 200 25 150 1.0 200 17.83 0.2377 0.512300 25 250 1.0 200 12.63 0.2269 0.523 200 50 100 1.0 200 21.28 0.13870.611 300 50 200 1.0 200 15.09 0.1248 0.625 400 50 300 1.0 500 23.320.1189 0.631 800 50 700 1.0 500 12.17 0.1109 0.639 400 100 200 1.0 50025.15 0.0693 0.681 800 100 600 1.0 500 13.15 0.0594 0.691 400 50 300 1.0750 34.97 0.1189 0.631 800 50 700 1.0 750 18.25 0.1109 0.639 400 100 2001.0 750 37.72 0.0693 0.681 800 100 600 1.0 750 19.73 0.0594 0.691 1000100 800 1.0 750 15.92 0.0578 0.692 1500 100 1300 1.0 750 10.73 0.05570.694 1000 200 600 1.0 750 16.5 0.0325 0.718 1500 200 1100 1.0 50 11.130.0300 0.720 1000 100 800 1.0 900 19.10 0.0578 0.692 1500 100 1300 1.0900 12.88 0.0557 0.694 1000 200 600 1.0 900 19.8 0.0325 0.718 1500 2001100 1.0 900 13.35 0.0300 0.720 Fibre resistance = 1 × 10⁻⁴ ohm m

1-17. (canceled) 18-29. (canceled)
 30. A fuel cell which comprises aplurality of first hollow electrically conductive gas permeable carbonfibres which are coated on the outside with a fuel cell catalyst andwhich have a means to enable a fuel vapour or gas to be passed down theinside of the fibre and a plurality of second hollow electricallyconductive gas permeable carbon fibres which are coated on the outsidewith a fuel cell catalyst and which have a means to enable oxygen or anoxygen containing gas to be passed down the inside of the fibber withthe first hollow electrically conductive gas permeable carbon fibresarranged side by side, so as to form a first sheet of the fibres, andwith the second hollow electrically conductive gas permeable carbonfibres arranged side by side to form a second sheet of the fibres, withthe fibres in each adjacent sheet at an angle to each other and in whichfirst electrically conductive has permeable fibber sheet (anode) and thesecond electrically conductive has permeable fibber sheet (cathode) areseparated by a fuel cell proton conducting layer which is positioned inbetween them.
 31. A fuel cell according to claim 30 in which theindividual fibers in each sheet are electrically connected to each otherat one or both ends.
 32. A fuel cell according to claim 31 in which theindividual fibers in each sheet are electrically connected by means of aconnecting block which contains a metal mesh that provides the means formaking an external connection to the cell.
 33. A fuel cell according toclaim 30 which comprises a multi-layered block.formed of a plurality ofalternate sheets in which the first sheet comprises a layer of the firstelectrically conductive gas permeable carbon fibres forming cathodes andthe second sheet comprises a layer of the second electrically conductivegas permeable carbon fibres forming the anodes with the fibres in eachalternate layer substantially orthogonal to each other.
 34. A fuel cellaccording to claim 30 in which the cathode fibres are connected togetherand the anode fibres are connected together in each block and aplurality of blocks are connected together in series and/or parallel.35-44. (canceled)
 45. A fuel cell which comprises two electrodesseparated by an electrolyte medium, wherein: (a) the first electrodecomprises at least one first hollow electrically conductive gaspermeable carbon fibre, the fibre consisting essentially of carbon whichis coated on the outside with a fuel cell catalyst and which has a meansto enable a fuel vapour or gas to be passed down the inside of the fibreand; (b) the second electrode comprises at least one second hollowelectrically conductive gas permeable carbon fibre, the fibre consistingessentially of carbon which is coated on the outside with a fuel cellcatalyst and which has a means to enable oxygen or an oxygen containinggas to be passed down the inside of the fibre; (c) the first and secondcarbon fibres being separated by the electrolyte medium.
 46. A fuel cellaccording to claim 45 in which the electrolyte medium is a solid polymerelectrolyte.
 47. A fuel cell according to claim 45 in which theelectrolyte medium is a liquid or gel electrolyte.
 48. A fuel cellaccording to claim 45 in which the electrically conductive gas permeablefibres in which sheet are aligned with their axes substantiallyparallel.
 49. A fuel cell according to claim 45 in which there iselectrically insulating mesh separating the first and second fibres. 50.A fuel cell according to claim 45 in which the outside diameter of theindividual hollow fibres is from 80 to 500 microns.
 51. A fuel cellaccording to claim 45 in which the wall thickness of the hollow carbonfibre is from 100-300 microns.
 52. A fuel cell according to claim 45 inwhich the wall thickness of the hollow carbon fibre is from 10-100microns.
 53. A fuel cell according to claim 45 in which the wallthickness of the hollow carbon fibre is from 20-100 microns.
 54. A fuelcell according to claim 55 in which the inside diameter of theindividual fibre is from 70 to 250 microns.